For the purpose of optimizing the efficiency of the catalytic converter (so-called three-way catalytic converter), which is effective for removing three noxious components (i.e., CO, HC, and NOx) contained in the exhaust gas of an internal combustion engine, it is necessary to maintain to or near the stoichiometric air-to-fuel ratio the air-to-fuel ratio of the exhaust gas supplied to the cylinders of the engine. Thus, irrespective of whether the fuel supply system of the engine employs a conventional caburetor or a fuel infector, the air-to-fuel ratio of the air-fuel mixture intake of the engine which utilizes the catalytic converter is controlled, in order to optimize the efficiency of the catalytic converter, by means of the feedback control upon the basis of the output of an air-to-fuel ratio sensor (so-called O.sub.2 (oxygen) sensor functioning upon the galvanic action due to the oxygen concentration) disposed, in the exhaust gas system of the engine.
Referring to FIGS. 1 and 2 of the drawings, let us briefly describe the conventional feedback control method of the air-to-fuel ratio utilizing an air-to-fuel ratio sensor; the method is disclosed, for example, in Japanese laid-open patent application No. 52-48738 or Japanese patent publication No. 62-12379. FIG. 1 shows the waveforms of the signals of the air-to-fuel ratio control system in the case where the rpm (revolution per minute) of the engine is relatively low; FIG. 2, on the other hand, shows the corresponding waveforms in the case where the rpm of the engine is relatively high. FIGS. 1(a) and 2(a) show the waveform of the output signal of the oxygen sensor which generates an output voltage corresponding to the O.sub.2 concentration of the exhaust gas; on the other hand, FIGS. 1(b) and 2(b) show the waveforms of the air-to-fuel ratio signals which are obtained by comparing the voltage signals of FIGS. 1(a) and 2(a), respectively, with a reference voltage of 0.5 V, and subjecting thereafter the resulting comparison signals to a waveform shaping; the air-to-fuel ratio is controlled by the proportional plus integral (PI) control method as indicated by the air-to-fuel control signals shown at FIGS. 1(c) and 2(c), which are obtained from the air-to-fuel ratio signals shown at FIGS. 1(b) and 2(b), respectively.
The air-to-fuel ratio of the engine is controlled by the feedback control method on the basis of the signals of FIGS. 1 and 2 as follows:
The air-to-fuel ratio sensor detects the O.sub.2 concentration contained within the exhaust gas of the engine; the output of the air-to-fuel sensor is utilized for the purpose of judging whether the air-to-fuel ratio of the air-fuel combustion mixture within the combustion chamber of the engine is smaller or greater than the stoichiometric air-to-fuel ratio, which is usually at about 14.7. (The state in which the air-to-fuel ratio is smaller than the stoichiometric ratio is referred to as the rich state, whereas that in which the air-to-fuel ratio is greater than the theoretical is referred to as the lean state.) The amount of fuel supplied to the engine, or the air-to-fuel ratio .lambda., is controlled upon the basis of the resulting comparison judgement signal, whose waveforms are shown in FIGS. 1(b) and 2(b); this feedback control of the air-to fuel ratio is effected as follows: First, referring to FIG. 1, let us consider the case where the rpm Ne of the engine is low. When the waveform of the comparison signal shown at FIG. 1(b) obtained from the output of the air-to-fuel ratio sensor shown at FIG. 1(a) is inverted from the lean to the rich state, the feedback control signal shown at FIG. 1(c) is skipped (or jumped), with a delay time D, to the direction of the lean side by an skipping amount B as the proportional feedback amount; thereafter, until the air-to-fuel ratio signal (i.e., the comparison signal) shown at FIG. 1(b) is inverted from the rich to the lean state, the air-to-fuel ratio signal is integrated with a negative constant multiplier, so as to obtain the feedback control signal shown at FIG. 1(c) which decreases linearly to the direction of the lean side with a constant negative slope C; when the air-to-fuel ratio comparison signal of FIG. 1(b) is inverted from the rich to the lean state, the control signal of FIG. 1(c) is skipped immediately by an amount B to the direction of the rich side; and after this inversion, until the air-to-fuel ratio signal is inverted again from the lean to the rich state, the control signal of FIG. 1(d) is obtained by integrating the air-to-fuel ratio signal of FIG. 1(b) toward the rich side to obtain the feedback control signal waveform curve with a positive slope C. The above operations are repeated to obtain the waveform of FIG. 1(c) from that of FIG. 1(b).
The method of control during the high rpm operation of the engine is similar to that during the low rpm operation: FIG. 2(b) shows the waveform of the air-to-fuel ratio comparison signal obtained from the output of the air-to-fuel ratio sensor when the rpm, Ne, of the engine is high; the waveform of the corresponding feedback control signal is shown at FIG. 2(c).
By the way, the delay time D is provided in the above control operation for the purpose of compensating for the detection response delay of the output of the air-to-fuel ratio sensor which detection delay takes place at times when the ambient atmospher around the air-to-fuel ratio sensor is inverted from the lean to the rich, or from the rich to the lean, state. It is noted that the lengths of the delay time D are illustrated longer than its true values; this exaggeration of the lengths of the delay D is for the purpose of explanation.
By means of the above control method, the average air-to-fuel ratio is controlled to the stoichiometric air-to-fuel ratio, so that the gas cleaning function of the catalyst converter rhodium is optimized.
The above control method, however, has the following disadvantages. Namely, as is apparent from FIGS. 1 and 2, both the control period T and the feedback control signal oscillation amplitude A are small when the engine is in the high rpm and high load region; on the other hand, both T and A become greater when the engine is in the low rpm and low load region. This is due to the fact that various transmission delay factors (which take greater values in the low rpm and low load region) exist between the point of time at which the inversion of the controlled (actual) air-to-fuel ratio takes place and the point of time at which the output of the air-to-fuel ratio sensor disposed at the exhaust gas system of the engine is inverted; namely, it takes a length of time for the air-to-fuel mixture introduced into the combustion chamber to be combusted therein and to be exhausted therefrom to reach the exhaust manifold at which the air-to-fuel ratio sensor is disposed. In the case where the transmission delay is great, as in the case of low rpm operation, shown in FIG. 1(c), there arises the phenomenon that when the air-to-fuel ratio comparison signal of FIG. 1(b) is inverted from the rich to the lean or from the lean to the rich side, the level of the control signal is not inverted with a skipping amount B toward the rich side; it is only inverted after an integration of the comparison signal over a length of time. Thus, the control period T increases the more due to the delay time that takes place between the time point of the inversion of the output of the air-to-fuel ratio sensor and the time point of the inversion of the feedback control signal (or the inversion of the manipulated variable), whereby the control signal oscillation amplitude A is further increased. As a result, the ensuing oscillation (hunting) of the engine during the idling period, etc., may give unpleasant feelings to the operator of the automotive vehicle.
A further disadvantage is this: Since the waveforms of the feedback control signal at the inversions from the lean to the rich, or from the rich to the lean side, are different according as the engine is in the low or in the high rpm region, the dispersion or scattering of the values of the detection response delay of the air-to-fuel ratio sensor itself that takes place at the inversions of the air-to-fuel ratio sensor is displaced by small amounts; this renders inappropriate the delay time D of FIG. 1 (which is provided for the purpose of compensating a predetermined level of the detection response delay) under certain operating conditions of the engine; thus, the controlled air-to-fuel ratio is deviated from the stoichiometric air-to-fuel ratio, thereby deteriorating the exhaust gas cleansing characteristics of the catalytic converter.
A still another problem of the conventional feedback control of the air-to-fuel ratio is this: due to the dispersion or scattering of the oxygen sensor characteristics or the temporal changes thereof, it is difficult to realize constantly the optimum cleaning characteristic which may be attained by each catalytic converter rhodium; thus, it becomes necessary to utilize a catalytic converter having an excessive capacity so as to allow for a certain allowance.